Artificial van der Waals heterostructures with two-dimensional (2D) atomic crystals are promising as an active channel or as a buffer contact layer for next-generation devices. However, genuine 2D heterostructure devices remain limited because of impurity-involved transfer process and metastable and inhomogeneous heterostructure formation. We used laser-induced phase patterning, a polymorph engineering, to fabricate an ohmic heterophase homojunction between semiconducting hexagonal (2H) and metallic monoclinic (1T') molybdenum ditelluride (MoTe2) that is stable up to 300°C and increases the carrier mobility of the MoTe2 transistor by a factor of about 50, while retaining a high on/off current ratio of 10(6). In situ scanning transmission electron microscopy results combined with theoretical calculations reveal that the Te vacancy triggers the local phase transition in MoTe2, achieving a true 2D device with an ohmic contact.

The excess of surface dangling bonds makes the formation of free-standing two-dimensional (2D) metals unstable and hence difficult to achieve. To date, only a few reports have demonstrated 2D metal formation over substrates. Here, we show a free-standing crystalline single-atom-thick layer of iron (Fe) using in situ low-voltage aberration-corrected transmission electron microscopy and supporting image simulations. First-principles calculations confirm enhanced magnetic properties for single-atom-thick 2D Fe membranes. This work could pave the way for new 2D structures to be formed in graphene membranes.

Despite numerous studies on two-dimensional van der Waals heterostructures, a full understanding of the charge transport and photoinduced current mechanisms in these structures, in particular, associated with charge depletion/inversion layers at the interface remains elusive. Here, we investigate transport properties of a prototype multilayer MoS/WSe heterojunction via a tunable charge inversion/depletion layer. A charge inversion layer was constructed at the surface of WSe due to its relatively low doping concentration compared to that of MoS, which can be tuned by the back-gate bias. The depletion region was limited within a few nanometers in the MoS side, while charges are fully depleted on the whole WSe side, which are determined by Raman spectroscopy and transport measurements. Charge transport through the heterojunction was influenced by the presence of the inversion layer and involves two regimes of tunneling and recombination. Furthermore, photocurrent measurements clearly revealed recombination and space-charge-limited behaviors, similar to those of the heterostructures built from organic semiconductors. This contributes to research of various other types of heterostructures and can be further applied for electronic and optoelectronic devices.

We propose a detailed mechanism for the growth of vertical graphene by plasma-enhanced vapor deposition. Different steps during growth including nucleation, growth, and completion of the free-standing two-dimensional structures are characterized and analyzed by transmission electron microscopy. The nucleation of vertical graphene growth is either from the buffer layer or from the surface of carbon onions. A continuum model based on the surface diffusion and moving boundary (mass flow) is developed to describe the intermediate states of the steps and the edges of graphene. The experimentally observed convergence tendency of the steps near the top edge can be explained by this model. We also observed the closure of the top edges that can possibly stop the growth. This two-dimensional vertical growth follows a self-nucleated, step-flow mode, explained for the first time.

non-wetting surface can enable the growth of perovskite films with large grain size and high crystallinity, which impressively suppresses the nonradiative recombination and improves the photovoltaic performance of PSCs. [23] Snaith and co-workers have used a self-assembled fullerene monolayer to grow high-quality perovskite films and passivate the defects on TiO 2 layers, leading to hysteresis-free PSCs with good photovoltaic performance. [25] Therefore, proper surface modification of substrates can decrease the density of nucleation sites and enlarge the grains of perovskite films. However, the orientation of the solution processed perovskite films cannot be conveniently controlled.Van der Waals (vdW) epitaxy is a prevalent technique for preparing high-quality semiconductor films with preferential orientations on 2D substrates with smooth and dangling-bond-free surfaces. [26][27][28] The weak vdW interactions between the crystals and the substrates can enable epitaxial growth of the films with high crystallographic orientation and low defect states even in the presence of large lattice mismatch and symmetry misfit between them. [29,30] Recently, Duan and co-workers realized scalable solution-phase vdW epitaxial growth of cubic PbSe layer on 2D rhombohedral Bi 2 Se 3 nanoplates. [31] Liu and co-workers also employed solution-phase vdW epitaxy strategy to fabricate ultrathin graphdiyne film with high quality by using 2D graphene as a growing template. [32] Inspired by the intriguing effects, we consider that dangling-bond-free 2D materials with proper lattice parameters can be utilized as growth templates for preparing high-quality perovskite films with a controllable orientation.Molybdenum disulfide (MoS 2 ) is a promising 2D material with high carrier mobilities and a suitable energy band structure for many optoelectronic applications. [33][34][35] Due to its dangling-bondfree and clean surface, MoS 2 has been used as growth templates for preparing vdW epitaxial 2D materials. [36] In this paper, solution processed large few-layer MoS 2 flakes have been employed as a growth template for perovskite films. We find for the first time the vdW epitaxial growth of MAPbI 3 perovskite on MoS 2 flakes, leading to highly oriented perovskite films with large grain sizes and low defect densities. Transmission electron micro scopy (TEM) images demonstrate that the (008) plane of MAPbI 3 and the (110) plane of MoS 2 can match perfectly, which facilities the out-of-plane growth of perovskite films with preferential orientation along (110). Then, MoS 2 flakes are modified on The quality of perovskite films is critical to the performance of perovskite solar cells. However, it is challenging to control the crystallinity and orientation of solution-processed perovskite films. Here, solution-phase van der Waals epitaxy growth of MAPbI 3 perovskite films on MoS 2 flakes is reported. Under transmission electron microscopy, in-plane coupling between the perovskite and the MoS 2 crystal lattices is observed, leading to perovskite films ...

Grain boundaries in monolayer transition metal dichalcogenides have unique atomic defect structures and band dispersion relations that depend on the inter-domain misorientation angle. Here, we explore misorientation angle-dependent electrical transport at grain boundaries in monolayer MoS2 by correlating the atomic defect structures of measured devices analysed with transmission electron microscopy and first-principles calculations. Transmission electron microscopy indicates that grain boundaries are primarily composed of 5–7 dislocation cores with periodicity and additional complex defects formed at high angles, obeying the classical low-angle theory for angles <22°. The inter-domain mobility is minimized for angles <9° and increases nonlinearly by two orders of magnitude before saturating at ∼16 cm2 V−1 s−1 around misorientation angle≈20°. This trend is explained via grain-boundary electrostatic barriers estimated from density functional calculations and experimental tunnelling barrier heights, which are ≈0.5 eV at low angles and ≈0.15 eV at high angles (≥20°).

Single-crystalline monolayer hexagonal WS is segmented into alternating triangular domains: sulfur-vacancy (SV)-rich and tungsten-vacancy (WV)-rich domains. The WV-rich domain with deep-trap states reveals an electron-dedoping effect, and the electron mobility and photoluminescence are lower than those of the SV-rich domain with shallow-donor states by one order of magnitude. The vacancy-induced strain and doping effects are investigated via Raman and scanning photoelectron microscopy.

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